This invention relates generally to electromagnetic flowmeters, and more particularly to a high-voltage impulse drive system for the excitation circuit of a meter of this type serving to effect a significant reduction in the power required to effect excitation.
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is obviated and the A-C flow induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that are most troublesome are:
1. stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and
2. induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the A-C flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow-induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow induced signal, thereby producing a changing zero shift effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic arrangement adapted to buck out this component, but elimination of its in-phase component has not been successful. Existing A-C operated electromagnet flowmeters are also known to vary their calibration as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltage both with respect to phase and magnitude.
Hence it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field, (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But, as previously noted, d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
In the patent to Mannherz et al., U.S. Pat. No. 3,783,687, whose entire disclosure is incorporated herein by reference, there is disclosed an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects.
In this prior patent, the driver system for exciting the coil includes switching means acting to periodically reverse the raw output of an unfiltered full-wave rectifier operated from an a-c power line. Because the electromagnet has a relatively high inductance, it functions as a filter choke which takes out a substantial percentage of the ripple component in the raw output of the rectifier, thereby obviating the need for filter capacitors. In This drive system, a logic circuit or divider is provided which is activated at the power line frequency (i.e., 50 or 60 Hz) to produce low frequency gating pulses for governing the electromagnetic reverse switching action.
Drive systems which are presently employed to provide excitation current for an electromagnetic flowmeter of the type disclosed in the Mannherz et al. patent utilize a constant-voltage drive. The long L/R time constant of the electromagnet produces a relatively slow magnet current rise time; hence a long excitation period is required to attain a constant flux level.
Because the total voltage and R are large, to reduce the magnet time constant to usable values, a substantial amount of power has to be dissipated by the drive system. As a consequence, a great amount of energy is lost in heat and the system is inefficient in power terms.